In integrated circuits, such as microprocessors, memories, and the like, signals may be routed for relatively long distances using transmission lines. A transmission line may be a bus, a printed circuit board trace, or other type of relatively long metal line for transporting a digital signal. Typically, a printed circuit board trace has a characteristic impedance of between 50 and 75 ohms. The receiving end of the transmission line is typically connected to an input of a logic circuit having a relatively high input impedance, where the input impedance is higher than the characteristic impedance of the transmission line. A buffer circuit is coupled to the sending end of the transmission line that typically has a very low impedance relative to the receiving end. A signal transmitted along the transmission line may be reflected back to the sending end, causing the signal to overshoot a planned steady state voltage for a particular logic state. The signal may be reflected back and forth many times between the receiving end and the sending end, causing oscillatory behavior of the signal at both ends. This repeated overshooting and undershooting of the signal is commonly known as "ringing" , and results in reduced noise immunity and increased time for the signal to become, and remain, valid at the receiving end.
In the past, one solution to overshoot has been to use clamping circuits to reduce the overshoot. However, clamping circuits may not respond fast enough to reduce overshoot caused by reflected signals. Also, if an integrated circuit at the sending end is supplied with a different power supply voltage (e.g. 5 volts) than the power supply voltage of an integrated circuit at the receiving end (e.g. 3 volts), the signal may be clamped at a voltage that is too high for the receiving end integrated circuit, causing reliability problems. In addition, if the receiving end of the transmission line is unterminated, or has a changing load, the signal voltage may increase to an unacceptably high voltage at the receiving end.